Most three-bladed and two-bladed turbines are equipped with a yaw system to be able to change the orientation of the rotor axis in order to follow the wind direction and ensure that the rotor faces into the wind. Most three-bladed and two-bladed turbines modulate the pitch of the rotor blades to control the speed of the turbine and hence the power output of the turbine, as well as for shutdown.
In two-bladed wind turbines, it is possible to yaw the turbine into or out of the wind also to control the speed of the turbine, and hence the power output of the turbine, without any need of pitch control. This is possible where the two-bladed turbine uses a teetering hinge (or “see-saw” hinge) to attach the turbine blades to the drive train of the turbine. The teetering hinge gives the rotor an additional degree of freedom, allowing the turbine to overcome gyroscopic forces, and to modulate the yaw angle sufficiently quickly to control the rotor speed of the turbine.
In turbines that use yaw control for controlling the running speed of the rotor, care must be taken to ensure that the teetering angle is controlled and kept within certain boundaries to avoid hitting the support structure and to ensure efficient dynamic performance of the rotor, particularly when undergoing fast yawing due to extreme wind conditions.